Polymers are materials having long chemical chains composed of many repeat units. Polymers are prepared using monomer units which undergo a chemical reaction resulting in formation of repeat chemical bonds arranged into long chain structures having relatively high molecular weights. These polymers can exist in a solid or liquid state and are typically called resins. Resins are then processed using techniques such as extrusion, molding, forming, and casting, to fabricate products with desired properties for various applications.
There are various types of polymer resins, often classified according to their polymerization chemistry and fabrication processes. Classifications include: thermoplastics which soften and flow when heated during processing, thermosets which undergo a chemical change during processing, and engineering resins that are processed in a nonconventional manner. Fabrication methods pertinent to polymer resins include: molding processes in which finely divided plastic is forced by the application of heat and pressure to flow into, fill, and conform to the shape of a cavity (mold); calendering process used for the manufacture of sheet or film, whereby granular resin is passed between pairs of highly polished heated rolls under high pressure; casting processes, in which fine particles of resin are suspended in a liquid medium that are then allowed to flow onto a support substrate or large polished wheel; extrusion processes, in which the polymer resin is propelled continuously along a cylindrical barrel under controlled shear conditions, for example with the aid of a screw motion through regions of high temperature and pressure or with the aid of a ram piston, through a preshaped die. A wide variety of shapes can be made by extrusion, including rods, sheets, channels, and tubes.
Some polymers are also suitable for post processing after fabrication. One example of a post fabrication process is expansion after extrusion, which results in porous, flexible articles. Polymers suitable for expansion (such as polytetrafluoroethylene (PTFE), ultra high molecular weight polyethylene (UHMWPE), and polyethyleneterephthalate (PET) ) are composed of long polymer chains. Chain length determines molecular weight, and chain orientation dictates crystallinity.
UHMWPE polymer resin is processed in a manner similar to PTFE, using preformed billets and ram extrusion, although it is not necessary to add an extrusion aid because the material is less shear sensitive, followed by expansion and sintering under applied heat and force.
PET polymer resin is a long chain, highly crystalline polymer, that is extruded using conventional extrusion techniques to form an extruded article. The extruded article may then be expanded and/or stretched at elevated temperatures.
A known method of forming an article made of PTFE is to blend a powdered resin with a lubricant or extrusion aid and compress the combination under relatively low pressure into a preformed billet. Using a ram-type extruder, the billet is then extruded through a die having a desired cross-section. Next, the lubricant is removed from the extruded billet by drying or by another extraction method. The dried extruded material (hereinafter xe2x80x9cextrudatexe2x80x9d), is then stretched and/or expanded at elevated temperatures below the crystalline melting point of the resin. In the case of PTFE, this results in the material taking on a microstructure characterized by elongated nodes interconnected by fibrils. Typically, the nodes are oriented with their elongated axis perpendicular to the direction of stretching.
After stretching, the extrudate is sintered by heating it to a temperature above its crystalline melting point while being maintained in a stretched condition. This can be considered an amorphous locking process for permanently xe2x80x9clocking-inxe2x80x9d the microstructure in the expanded or stretched configuration.
Sometimes it may be desirable to modify the surface characteristics of articles made of PTFE. Conventional surface treatment approaches have been developed for modifying the surface characteristics of PTFE extruded substrates. According to one method, glow discharge plasma techniques, such as Radio Glow Discharge (RGD), are used to perform the surface modifications. Those surface modifications include plasma polymerization, plasma activation and plasma etching. Plasma polymerization entails using radio frequency gas plasma and polymerizing gases to polymerize a material onto a substrate surface. Plasma activation entails using a non-polymer forming gas, such as oxygen or a saturated fluorocarbon, to chemically modify a substrate surface. Plasma etching techniques employ reactive gas plasma to etch or roughen a surface by removing quantities of the substrate surface material. Etching can also be accomplished with other energy sources such as ion beams. Additionally, conventional masking techniques can be used in combination with etching to produce a desired textured pattern.
Prior publications directed toward surface treatments disclose a variety of motivations for performing surface modifications. By way of example, some prior approaches are directed to enhancing biocompatability, non-thrombogenic properties, wettability, adhesiveness, hydrophobicity, cleanliness and/or bacteriacidal properties of the polymeric substrate surface. Surface treatments are also employed to alter the porosity, permeability, or chemistry of a substrate surface region.
A drawback of conventional surface treatment approaches is that they operate on finished, fabricated and/or finally processed materials, thus rendering such approaches ineffective with regard to modifying bulk substrate properties, such as porosity and permeability. Additionally, chemistry modifications are limited to surface effects, as well as being limited to treating an entire article. As used herein, the term xe2x80x9cchemistryxe2x80x9d refers to the atomic elements that comprise particular materials, along with the concentration of each element included in the particular material.
A typical application for substrates having regions of selective porosity and chemistry characteristics the fabrication of vascular grafts. By way of example, it is sometimes desirable to fabricate grafts that are relatively porous on an outer surface to encourage tissue ingrowth and anchoring, but relatively nonporous on an inner surface so as not to promote thrombosis or leakage.
One conventional technique for tailoring porosity involves employing non-uniform lubrication levels in a preform. Other conventional approaches involve stacking preforms of different PTFE materials, PTFE and a dissimilar material, or preforms fabricated with different lubrication levels together and extruding a structure. Another prior approach is directed to surrounding an inner extruded PTFE tube with one or more additional concentric layers of tubing having selected porosities. Other conventional methods for varying substrate porosity attempt to modify the characteristics of polymeric resins. One such prior art method, irradiates a polymer powder resin with ionizing radiation, prior to compressing the resin into a billet. According to that method, polymeric powder resin is exposed to ionizing radiation in the range of 0.01-2 Mrad. As a result, the polymeric powder exhibits improved powder flow properties, and when combined with lubricants requires lower pressure to extrude the resultant paste then does paste formed with untreated resin. This method also discloses combining the treated powder with untreated resin and/or silica to achieve a variety of extrusion pressures and flow properties.
Another conventional approach for varying substrate porosity irradiates PTFE scraps at an energy dose ranging from 10-1000 kGy, where a Gy is an SI unit and is equivalent to a joule/kg. The irradiation process degrades the PTFE to have a relatively low average molecular weight of less than 106. The process also lowers the melting temperature and reduces the particle size of the PTFE resin to range from 0.1-100 micrometers. The radiation-degraded PTFE can be combined with untreated PTFE, having a relatively higher molecular weight in the range of 1xc3x97106 to 50xc3x97106.
A drawback of conventional pre-extrusion irradiation methods is that they degrade the PTFE by reducing the molecular weight and particle size. While these methods produce materials that can be mixed with untreated PTFE, they fail to produce stand alone materials capable of extrusion.
The present invention is directed generally to methods for treating polymer resins and products produced therefrom. More particularly, the invention relates to methods for treating polymer resins prior to subsequent processing of hose resins, and to articles fabricated from polymer materials so treated. According to an illustrative process of the invention, a resin of an expandable polymer such as, polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxytetrafluoroethylene (PFA), ultra high mollecular weight polyethelene (UHMWPE) or polyetheleneterephthalate (PET)is provided. The resin is then treated with a plasma energy such as plasma glow discharge treatment, to a sufficient degree that the treatment effects a porosity, and/or a chemistry quality in articles fabricated from the treated resin. According to other embodiments, the plasma treatment effects other bulk properties of the resin, such as permeability, fibril density, and node size of articles fabricated from the treated resin.
According to one aspect of the invention, the plasma glow discharge treatment includes plasma polymerization onto the surface of a polymer resin. n an alternative embodiment, the plasma glow discharge treatment includes plasma activation of the resin. In a further embodiment of the invention, the plasma glow discharge treatment includes plasma etching of the resin. In one embodiment, the plasma glow discharge treatment includes a Radio Glow Discharge (RGD) treatment.
According to a further feature, a process of the invention employs the plasma energy treated resin in articles of manufacture. An illustrative process for incorporating the treated resin into an article of manufacture includes the steps of combining the treated resin with an extrusion aid to form a blended polymer paste, compressing the blended paste into a preformed billet, extruding the preformed billet into an extruded shape, and expanding and sintering the extruded article. According to a further feature, an illustrative process of the invention forms an implantable prosthesis from the sintered preformed billet. According to additional features, the implantable prosthesis may be, among other devices, vascular grafts, endovascular liners and grafts, prosthetic patches, vascular access devices, shunts, catheters, sutures or implantable tissue augmentation devices, such as those used in cosmetic surgery. According to yet a further feature, the articles of manufacture include single and multilayered membranes. Such membranes may be employed in clinical diagnostic test strips or in filtration devices.
According to one aspect of the invention, the polymer resin is subjected to the plasma discharge treatment. However, according to other embodiments, the blended paste formed from the polymer resin and the extrusion aid is subjected to the plasma discharge treatment. In alternative embodiments, preformed paste billets and unexpanded extrudates are subjected to the plasma treatment. As used herein, the term component refers to untreated polymer resins, pastes, preformed billets, and unexpanded extrudates formed from expandable polymers. According to other features of the invention, the blended treated pastes, preformed billets or unexpanded extrudates that are treated with plasma discharge are employed in fabricating articles, such as those mentioned above.
According to another aspect of the invention, polymer articles formed in accord with processes of the invention have regions of differing bulk characteristics, such as porosity, fibril density, node size (referred to collectively herein as porosity qualities), permeability, and chemistry. According to one embodiment, differently plasma discharge treated pastes, or pastes formed from differently treated resins are disposed in selected regions of a preform mold and compressed into a preformed billet. As a result, the subsequently extruded billet includes a plurality of regions having different bulk characteristics. In a related embodiment, differently treated pastes, or pastes formed from differently treated resins are disposed in preform molds having a plurality of chambers separated by discrete barriers. The chambers facilitate the formation of billets having configurable regions of selected bulk properties. According to a further feature of the invention, instead of being created by discrete barriers, the different regions having different bulk properties are merged through gradients, such as porosity gradients.
In an alternative embodiment, methods of the invention provide an improved polymer resin by introducing a polymer resin, such as PTFE, FEP, PFA, PET or UHMWPE resin, into a chamber and plasma glow discharge treating the resin. According to another embodiment, a method of the invention provides an improved blended paste by blending a resin of a polymer, such as PTFE, FEP, PFA, PET or UHMWPE, with an extrusion aid, such as ISOPAR-G or ISOPAR-H, available from Exxon Corporation, and plasma glow discharge treating the blended paste.
According to further aspects, the invention provides a variety of articles of manufacture, such as prosthetic devices and membranes, having selectable porosity and chemistry characteristics, and formed from polymer resins, pastes, preformed billets and unexpanded extrudates, each being treated with a plasma discharge.